In U.S. Pat. No. 3,434,774, it is disclosed that more efficient transmission of optical wave energy along a multimode optical fiber is achieved by grading the refractive index of the fiber core. The degree of transmission efficiency achievable with a particular fiber is determined by how closely the fiber's index profile approaches the optimum distribution. Thus, accurate knowledge of the fiber profile is necessary in order to assess the fiber's transmission properties. Inasmuch as a fiber is a reduced replica of the preform from which it is drawn, the profile information sought can just as readily be obtained by measuring it in the preform. Indeed, there are a number of advantages in doing so. Its larger diameter and shorter length make it easier to handle, and permit easy examination over its entire length. Furthermore, a poor preform can be discarded without having incurred the expense of drawing the fiber only to find that the fiber is useless for its intended purpose.
One technique for examining preforms, using the ultraviolet-fluorescence method, is disclosed in U.S. Pat. No. 4,161,656 by D. Marcuse et al. This method is based upon the discovery that the index-modifying dopants used to grade the index profile can be made to fluoresce when illuminated with ultraviolet radiation. In particular, the intensity of this fluorescence is directly proportional to the dopant concentration which, in turn, is proportional to the refractive index. Accordingly, as taught in the above-cited Marcuse et al patent, the dopant concentration is determined by uniformly illuminating the entire width of the fiber preform with uv light and measuring the distribution of the resulting fluorescence along a direction transverse to the longitudinal axis of the preform. However, because the measured fluorescence includes components contributed from throughout the cross section of the core, mathematical manipulation of the data is necessary in order to extract the desired information. Indeed, as a practical matter, a computer is required for these calculations.